As a water-soluble carboxyl group-containing polymer, there have been known, for instance, a copolymer of an α,β-unsaturated carboxylic acid such as acrylic acid and a polyallyl ether (U.S. Pat. No. 2,923,692); a copolymer of an α,β-unsaturated carboxylic acid and hexaallyl trimethylene trisulfone (U.S. Pat. No. 2,958,679); a copolymer of an α,β-unsaturated carboxylic acid and triallyl phosphate (U.S. Pat. No. 3,426,004); a copolymer of an α,β-unsaturated carboxylic acid and glycidyl methacrylate (Japanese Patent Laid-Open No. Sho 58-84819); a copolymer of an α,β-unsaturated carboxylic acid such as acrylic acid and pentaerythritol allyl ether (U.S. Pat. No. 5,342,911, No. 5,663,253 and No. 4,996,274); a copolymer of an α,β-unsaturated carboxylic acid such as acrylic acid, a (meth)acrylic acid ester and pentaerythritol allyl ether (Japanese Examined Patent Publication No. Hei 5-39966 and No. Sho 60-12361); and the like. These water-soluble carboxyl group-containing polymers have been used for uses such as a thickener for cosmetics, a moisturizer for poultices and the like, an emulsifier, or a suspension stabilizer for suspensions and the like, after these water-soluble carboxyl group-containing polymers are dissolved in water and the solution is neutralized with an alkali, to give a neutralized viscous liquid.
In order to use the above-mentioned water-soluble carboxyl group-containing polymer for these uses, it is necessary to prepare a neutralized viscous liquid by firstly preparing a homogeneous aqueous solution of the water-soluble carboxyl group-containing polymer, and thereafter neutralizing the aqueous solution with an alkali. However, since the above-mentioned water-soluble carboxyl group-containing polymer is usually in the form of fine powder, a lump of powder tends to be formed when the polymer is dissolved in water. Once the lump of powder is formed, a gel-like layer is formed on the surface of the lump. Therefore, there is a defect that the speed of penetrating water into the interior of the lump is delayed, thereby making it difficult to obtain a homogeneous aqueous solution.
Therefore, when the aqueous solution of the water-soluble carboxyl group-containing polymer is prepared, there is necessitated a procedure with a poor production efficiency such that powder of the water-soluble carboxyl group-containing polymer is gradually added to water under high-speed agitation in order to prevent the formation of the lump of powder. In some cases, there is necessitated a specialized dissolution apparatus in order to prevent the formation of the lump of powder.
On the other hand, in the field of cosmetics and the like, the above-mentioned water-soluble carboxyl group-containing polymer is dissolved under a high-speed agitation, and thereafter the solution is neutralized with an alkali to give a neutralized viscous liquid, and a high-speed agitation is further continuously carried out in order to obtain a smoother gel appearance of a neutralized viscous liquid. However, when the high-speed agitation is maintained for a long period of time, the viscosity of the neutralized viscous liquid is reduced due to shearing during the agitation. Therefore, there is a defect that the above-mentioned water-soluble carboxyl group-containing polymer has to be used in a large amount for obtaining the same thickening effect.
An object of the present invention is to provide a process for preparing a water-soluble carboxyl group-containing polymer showing a small reducing ratio of viscosity when prepared its neutralized viscous liquid and a high viscosity even at a low concentration, and being excellent in transparency and gel appearance (surface smoothness).
These and other objects of the present invention will be apparent from the following description.